Method of separating gaseous mixtures



July l, 1958 w. E. LOBO E-rAl. 2,840,994

METHOD OF SEPARATING GASEOUS MIXTURES Filed Jan. 3.1. 1947 s sheets-sheet 1 ATTQRNEY July 1, 195,8 -w. E. LoB Erm.

METHOD 0F SEPARATING` GASEOUS MIXTURES 3 Sheets-Sheet 2 Filed Jan. 3l. 1947 www M. @PlSSN @QN mmm.

Mug/@2&0 @50H65 7.' SHA PERLA S ATTRNEY July 1, 1958 w. E. LoBo r-:rAL

METHOD OF SEPARATING GASEOUS MIXTURES 5 Sheets-Sheet 3 Filed Jan. 3l. 1947 United States Patent O METHOD OF SEPARATINGGASEOUS lVIlXTURES Walter E. Lobo, Westfield, N. J., and George T. Skaperdas, New York, N. Y., assignorsto The M. W. Kellogg Company, JerseyCity, N. J., a corporation of Delaware Application January 31, 1947, Serial No. 725,514

8 Claims. (Cl. 62-14) This invention relates to improvements in separating gaseous mixtures by liquefaction and fractionation. More particularly, the invention relates to low temperature gas separations wherein compressed gas mixtures are cooled in reversing heat-exchange zones by backward-returning expanded gaseous product, with deposition by precipitation of a higher boiling component and subsequent removal thereof by evaporation into the gaseous product, before a part of the cooled gas mixture is expanded to lower pressure in an expansion engine. Still more particularly, the invention relates to such separations in which additional heat exchange relationship is provided in a colder portion of the reversing heat-exchange zone, which includes the zone where precipitation and evaporation of the higher boiling component occurs, to maintain a difference between the temperatures of the compressed gas mixture and expanded gaseous product in said portion ofthe heat-exchange zone less than the maximum difference required for operating the heat-exchange zone without additively accumulating deposits of the higher boiling component therein.

When gas mixtures, for example air, have been separated by liquefaction and fractionation at low temperatures, a method for obtaining the required low temperatures has been to compress the incoming gaseous feed mixture and to precool it in a reversing countercurrent heat exchange step by heat exchange with cold product and thereafter expand a part of the cooled compressed gas with the performance of external work. During the precooling step, higher boiling components are precipitated as impurities from the gaseous feed mixture and deposited in the reversing heat-exchange zone. For example, in the case of air as the gaseous mixture, such components will be water and carbon dioxide. The deposits are then removed by the evaporating and scavenging action of a stream of at least one cold gaseous product passing through the heat-exchange zone in alternating, or interchanging, paths of flow with the gaseous feed mixture. However, for complete removal of such deposit in this manner, careful regulation of conditions in the zone of alternate precipitation and evaporation is necessary.

A reversing heat-exchange zone is periodically cleared of deposit if the temperature of the gas precipitating and the temperature of the gas evaporating each and every portion of higher boiling component are maintained sufficiently close to each other. The difference between such temperatures which is maximum in magnitude for periodically clearing the heat-exchange zone of deposit is defined herein as the maximum allowable temperature diterence. In any given operation, the proper tempera- -ture relationships can be reached by any suitable additional heat exchange but, conveniently, either with a separate stream of a portion of the cooled gas mixture, or by passing at least a part of a product gas first as a separate stream in heat exchange with the compressed gas passing through the zone of precipitation, or both.

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In this specification, the expression unbalance is used to define the above described procedure by which conditions are reached to maintain the difference between the temperature of precipitation and evaporation of each portion of higher boiling component sufficiently less than the maximum allowable temperature difference. Likewise, the separate stream which is used to provide the additional heat exchange for unbalance is termed the unbalance stream.

In order to provide for the substantially complete precipitation of higher boiling component during the precooling of the compressed gaseous feed mixture, the reversing heat-exchange zone, particularly when operating with the unbalance procedure, should be operated with its cold end temperature below the precipitating temperature of the component. This condition, however, causes the temperature of the gaseous mixture leaving the heatexchange zone to be so low that when a part of it is subsequently expanded at least some liquefaction occurs in the expansion engine. This results in reducing the efficiency of the expansion engine and creating operating diiculties. For instance, inasmuch as the expansion engine usually is located somewhat below the point of introduction of the expanded gas into the fractionator, the occurrence of liquefaction will cause an accumulation of liquid in the vertical rising line between the engine and fractionator, resulting in surging and uneven iiow therein. Consequently, various expedients have been resorted to heretofore to obviate this diiculty. For example, according to one method the cold end temperature of the heat-exchange zone is maintained at a` temperature above that which creates liquefying conditions in the expansion engine during expansion. This permits part of the higher boiling component to precipitate in the heat-exchange zone. Unprecipitated higher boiling component escaping from the heat-exchange zone, then precipitates in colder zones of the gas separation system and is removed by filtration.

It is the principal object of this invention to provide for a temperature of the -compressed gaseous mixture about to be expanded in an expansion engine that is suitable for operating the engine with its maximum eldciency by avoiding the formation of liquid during the expansion while simultaneously providing for operating the reversing heat-exchange zone with a minimum tem- .perature which is suiciently low to cause substantially the complete precipitation and deposition of higher boiling component therein. Other objects will be made apparent by the following detailed description of the invention.

In its preferred embodiment the invention involves, as an important step, the utilization of the heat absorbed by the unbalance stream to adjust the temperature of the stream of gaseous mixture about to be expanded in the expansion engine so that the temperature of this stream is higher than the average temperature of the main stream of cooled compressed gaseous mixture as it leaves the reversing heat-exchange zone. In this embodiment, the temperature of the gaseous mixture about to be expanded likewise will be higher than the average temperature of the gaseous mixture which is passed from the reversing heat-exchange zone into the fractionation step.

In the following detailed description of the invention, reference will be made to atmospheric air as illustrative of a gaseous mixture in the separation of which the present invention is applicable. It is to be understood, however, that the invention is applicable to the separation of other gaseous mixtures containing undesired higher boiling components as, for example, low molecular weight hydrocarbons.

The various arrangements `for. accomplishing the .foregoing temperature adjustments will be described by reference to the accompanying drawings inwhich:

Figure 1 is a diagrammatic representation irl-elevation Y of an arrangement of apparatus adapted to carry out the improved method in a process for the separation of atmospheric air by liquefaction and fractionation at low temperatures and under moderate superatmospheric pressure, wherein the additional heat exchange for unbalance is effected with a portion of the cooled compressed air.

Figures 2, 3 and 4 are diagrammatic representations of yarrangements of apparatus adapted to carry out modications of the method of the invention with respect to the processing arrangement shown by Figure 1.

Figure 5 is a diagrammatic representation inelevation of an arrangement of apparatus adapted to carry out the improved method of the invention in a process for the separation of atmospheric air by liquefaction and fractiOnation at low temperatures and under moderate superatmospheric pressure, wherein the additional heat exchange for unbalance is eiected by a portion of the nitrogen-rich product stream, the whole of which acts as the scavenging stream in the reversing heat-exchange zone.

Referring now to Figure l, an important function in the processing arrangement shown by this drawing is performed in reversing heat exchanger 3, which provides a zone for precooling the compressed feed air by countercurrent heat exchange with the cold product streams of oxygen-rich and nitrogen-rich components resulting from the separation of the air. While the principal function of heat exchanger 3 is the precooling of the compressed feed air, the exchanger is made to serve also as a zone of purication. That is, water and carbon dioxide, constituents that are usually associated with atmospheric. air drawn as feed into separation processes of this character, are precipitated from the air at the low temperatures to which the air is cooled and are left as liquid or. solid deposits on the metallic surfaces of the exchanger. Thus, not only cooled but purified air leaves the heat exchanger. Heat exchanger 3 consists of a multi-stream arrangement comprising four passageways for carrying air, nitrogen-rich and oxygen-rich products and an unbalance stream. Passageways 5 and 6 of the exchanger are "reversing passages which alternately carry compressed air and nitrogen-rich product in countercurrent heat exchange with each other. The passageways are similar in flow resistance and extend the full length of exchanger 3.

Passageway 16, carrying the oxygen-rich product countercurrently to the flow of the compressed air, likewise extends over the whole length of the exchanger, but the fourth passageway 42, called the unbalance passage, is shorter and usually is incorporated only into the colder section of the exchanger, as exemplilied in the drawing of Figure 1. The exchanger is shown in Figure l to diagrammatically represent in sectional elevation, a multiannular heat exchanger constructed of a number of Vcon-Y centric annular passageways surrounding -a central tu bular passageway. This particular construction of the exchanger is not essential as other forms of construction are just as applicable to the performance of the function of this apparatus. It is desirable, however, that the two similar passageways, which are employed for reversing, particularly be packed with a metallic material. The packing material may be of any suitable character andrconveniently may consist of a coil-of edgewound metallic ribbon, pins, longitudinally placed strips of metal, or the like. It is preferable that the metallic packing be adxed to they walls of the passageways with a suitable metal to metal bonding material, which may be a solder, to provide an efficient path of thermal flow between the fluid streams passing through the exchanger. In -the event that the passageways have not common.

boundary walls as in the type of construction in the iig-` ure, theseveral passagewayspreferably are also metal bonded to each other. Although heat exchanger 3 is shown in Figure 1 to represent a countercurrent heat exchanger, it is to be understood that the invention is not limited to this type of heat-exchange zone since it is equallyv applicable to processing arrangements involving the use of regenerative type heat-exchange zones and to the use of separate heat-exchange zones for exchanging heat between separate portions of the compressed air and each of the product streams.

In the normal operation of heat exchanger 3, a stream of atmospheric air is introduced under a moderate superatmospheric pressure and at about the temperatureof-the atmosphere through line 1 and valve 2 and passed to the exchanger by way of either line 4 or line 5. For example, the air is compressed to a pressure of the order of magnitude of 77 pounds per square inch gauge and aftercooled to a temperature approximately about F. before being introduced into line 1. To direct the ow of the incoming compressed feed air alternately into line 4 and line 5 at frequent periodic intervals of time, usually of about three minutes duration, the construction of valve 2 is of the reversing type. That is, the valve has a single inlet opening to the flow of the incoming feed air and two outlet openings, one leading into line 4 and the other leading to line 5, with suitable internal mechanism, the function of which is to direct the flowing air into either one of the two outlet connecting lines. Preferably, valve 2 is operated periodically by an automatic timing device so that the valve settings are automatically changed to divert the feed air alternately into line 4 or line 5 at the desired intervals of time. Reversing valve 9 is mechanically arranged to co-operate simultaneously.

with the action of valve 2. It is the function of this valve to direct the flow of backward-returning nitrogen-rich product, that also is passing alternately through lines 4 and 5, from these lines into the two outlet openings of valve 9, by way of connecting lines 8 and 15 and out the single outlet opening of the valve into exit line 10.

Before compression of the atmospheric feed air, or at least before the stream of air is drawn into line 1, it is desirable to treat the air to remove impurities, such as dust-and part of its water vapor content. As a further purifying treatment, it may be desirable to chemically eliminate all traces of acetylene which are usually present in atmospheric air. This chemical treatment may ybe accomplished in any desired manner but conveniently is performed by subjecting the air to the catalytic action of4 a suitable catalyst as, for example, one containing a mixture of copper and manganese oxides.

The incoming compressed air passes alternately from lines 4 and 5 into the warm end of the annular passageways 6 and 7 and in passing therethrough, is cooled in counterflow relation with cold nitrogen-rich product, being passed alternately through these same passageways.

Simultaneously, with its exchange of heat with the nitro-- gen-rich product the compressed air also is exchanging heatvcountercurrently with the oxygen-rich product flowing through passageways 16. As the air is cooled, water, ice and carbon dioxide are precipitated and deposited in the exchanger. Were the ow of air and nitrogen-rich product not interchanged between these passageways, the accumulation of ice and carbon dioxide would eventually plug the exchanger. However, reversing of valve 2 periodically diverts the air into the alternate passageway which has been carrying nitrogen-rich product and this change in flow causes check valves 11, 12, 13 and 14 to respond automatically to the change so that the nitrogenrich product is immediately changed from the passageway which has been carrying it into the passageway which has just been carrying the air. The streams of gaseous material in either reversing passageway thus are interchanged periodically by valve 2, but the ow of eachstream is always in the same direction. Because these two streams are in counterfiow, however, the direction of ow of gas relative to the deposited components is reversed upon action of valve 2 and, in consequence, the exchanger 3 is called a reversing exchanger and passageways 6 and 7 are designated as reversing passageways.

Inasmuchas the nitrogen-rich product, or scavenging stream is a resultant. product of the separation of air after it has been expanded, this stream is at a lower pressure than the incoming stream of compressed air with which it is exchanging heat in the reversing passageways Hence, the capacity of the scavenging stream to hold water or carbon dioxide in the vapor state is larger than the capacity of the air stream to do this at the same temperature. Therefore, as the scavenging stream passes over the deposit which the cooling of the air has left in the exchanger, such deposit is evaporated into the nitrogenrich product stream and carried out of the exchanger. In this manner the compressed air, as it leaves passageways 6 and 7 by way of lines 17 and 106 respectively, is in a cold, Vpuried condition, and as the nitrogen-rich product stream evaporates and removes deposited material from the exchanger, the foregoing cycle is capable of being repeated indelinitely if the evaporation is complete in each cycle. It is because the nitrogenrich product stream thus purges the exchanger of precipitated deposit, that it is deiined herein as the scavenging stream.

ln order to operate reversing heat exchanger 3 to remove precipitated deposit completely, it is necessary to establish operating conditions which will insure complete evaporation of such deposit. Otherwise, there will be an accumulation of deposit remaining unevaporated after each cycle which will gradually build up until eventually it plugs the exchanger. For a better understanding of the operation of the reversing exchanger and of the deposition and evaporation of carbon dioxide, it is convenient to consider, for exemplilication, a sectional length of one passageway that includes the cold end of the exchanger operating so that a relatively negligible quantity of this higher boiling component'leaves the exchanger in the cooled air. The amount of carbon dioxide entering and left in this section is then established from the ilow rate, the pressure and the temperature of the air entering the section, inasmuch as the air being cooled necessarily will be saturated at its entering temperature. ln order for carbon dioxide not to accumulate in the section, the same quantity of this component must be contained in the scavenging stream and, since the iiow rate of this stream is known, the actual concentration of carbon dioxide in this stream is established for the condition of complete evaporation.

It is well known that for any given consideration of a vaporized component in a gas, and for any given pressure of that gas, there exists a saturation temperature above which the gas will retain all of the component in the vapor pulse but below which the gas cannot contain the given concentration of vaporized component. The foregoing is true with respect to any given concentration of evaporated carbon dioxide held in the vapor phase by the gaseous scavenging stream. Therefore, if the scavenging stream leaving the section of the exchanger under consideration is colder than the saturation temperature corresponding to the above mentioned actual concentration of carbon dioxide resulting from complete evaporation ot this component and to the pressure of the scavenging stream, this stream will not be able to evaporate completely all or' the carbon dioxide deposited in the section in the previous period between reversals of Valve 2. It is apparent then that the scavenging stream must have a temperature equal to or greater than the established saturation temperature. Because the scavenging stream is at a lower pressure than the compressed air stream in the exchanger, it also is apparent that the saturation temperature of the scavenging stream must be lower than the saturation temperature of the compressed The difference between th.: temperature of the compressed air stream entering the sectional length of the exchanger under consideration and the saturation temperature of the scavenging stream leaving this section is a critical value because any actual operating differences between these temperatures in excess of this critical value indicate a scavenging stream which is too cold to evaporate the deposit of carbon dioxide completely and, ther-etere, indicates an accumulation of carbon dioxide deposit. ln other words, the foregoing condition indicates an inoperable exchanger because it will plug up by deposits of carbon dioxide. Temperature differences below this critical value indicate a scavenging stream which is at a temperature warmer than the saturation temperature, and which can, therefore, evaporate the carbon dioxide deposit completely. It is understood that to allow for factors which aifect the practical operation, such as incomplete saturation of the scavenging stream in its passage through the exchanger, it is desirable to operate with temperature diierences smaller than the critical value. As stated heretofore, this critical value has been deined as the maximum allowable temperature diiference, and is applicable to the point where the compressed air entered the sectional length of the exchanger selected for this exemplification, and to the temperature condition at that point. For other sectional lengths; similar maximum allowable temperature dilerences may be established corresponding to other temperature conditions, and the maximum allowable temperature difference decreases toward the colder portions of the exchanger.

Because the quantity of carbon dioxide that the scavenging stream can evaporate decreases with its temperature but increases with aldecrease in pressure, there are, therefore, two competing inluences involved in operation of exchanger S-the difference between the pressures of the countertlowing streams which aids evaporation and the dilterence between their temperatures which hinders evaporation, the resultant eect of which determines the actual evaporation. In process arrangements, such as is exemplified by Figure l, the difference between the pressure of the compressed air and the pressures of the products of the separation normally must be predeterminedand lixed by the refrigeration and distillation requirements that have to be decided upon and established within lixed limits at the time the process is designed. With one of the aforementioned competing inliuences involved in the operation of exchanger 3 thus iixed, it becomes only necessary to operate the reversing streams of this heat-exchange zone at temperatures such that the difference between these temperatures is less than the maximum allowable temperature difference to achieve continuously the complete evaporation and removal of carbon dioxide deposit in the interval of time between the change of setting of reversing valves 2 and 9. lt is to be understood that while the foregoing explanation has related to the removal of carbon dioxide only, the principles involved are equally applicable to the precipitation and evaporation of water or other relatively higher boiling components which may be present.

' The maximum allowable temperature difference which has been defined establishes the conditions within which the actual operation of reversing heat exchangers are practical for preciptation and evaporation. However, such conditions are not necessarily always obtainable. For example, in the separation of air by the process arrangement shown in Figure l when passageway 42 is not employed, the differences between the temperatures of the compressed air and the nitrogen-rich product streams do not remain less than the maximum allowable temperature difference over the whole length of exchanger 3. This tact results from the manner in which the specilic heats of the components of the reversing streams change with pressure. For example, the specific heat of air under a pressure of about 77 pounds per square inch gauge is,.somewhat larger than the specic heat of air or of its components ,at atmospheric pressure. Furthermore, the difference is smaller at the temperature of the atmosphere andincreases `moreand Vmore rapidly as the temperature drops. When passageway 42 is not employed, reversing exchanger 3 ,is said to operate in balanced flow because then the mass rate of flow, that is, the flow expressed in terms of weight per unit time, of the cornpressed air is equal to the sum of the mass rates of flow of the productsrof the separation. In such balanced llow heat exchange, the change in temperature of the stream. of higher specific heat is smaller than that of the streams of lower specific heat. As a result, because the compressed air has the higher specific heat in exchanger 3, Vthe difference between the temperatures of the reversingY streams increasestoward the cold end of the exchangerwhen balanced flow is used in the absence of passageway 42. In consequence of the progressively larger difference in specific heats as the temperature decreases, the difference-between the temperatures of the reversing streams in exchanger 3 increases more and more rapidly toward the cold end of the exchanger. This progressive change in the temperature difference relationship is of fundamental importance in the operation of the exchangers reversing passageways 6 and 7 because the ditferen-ce between the temperatures of the streams at the warm end of exchanger 3 is below the maximum allowable temperature difference for the evaporation of water and ice by the nitrogen-rich stream. Therefore, the water which has been precipitated during the cooling of the air, either as a liquid or as ice, readily c an be completely evaporated during the period between reversals of valves 2 and 9. Toward the cold end of exchanger 3, however, when passageway 42 is not employed, difference between the temperatures of the compressed air and the nitrogen-rich streams increases to values greater than the `critical value required for the complete evaporation of both carbon dioxide and water in the interval between the reversals of valves 2 and 9 and, therefore, the exchanger will tend to become inoperable. In this manner, the change in specific heat causes the difference between the temperatures of the compressed air and nitrogen-rich product to increase consistently towards the cold end of the exchanger to values that will make the exchanger normally inoperable when it is operating with balanced ow conditions. This inoperable condition is wholly independent of the difference between the temperatures of the streams at the warm end of exchanger 3 because even if this difference were negligible, the difference between the temperatures at the cold end of the exchanger would be in excess of the maximum allowable temperature difference.

Figure l shows an arrangement of one modication of a method for controlling temperature difference as employed in the separation of air. According to this modification, the compressed air when it leaves exchanger 3 through lines 17 or 106 and passes by way of check valves 13 and 14 into line 18 is at a temperature of about 273 F. From line 18 the air is passed through lter 19 by way of line 20, having valve 21 positioned therein. This filter contains a body of adsorbent material, such as silica gel or activated carbon particles, the function of which is to screen out any impurity such as a hydrocarbon or carbon dioxide snow which may have penetrated thus far through the system. The lter is, of course, especially useful during starting-up periods. The cooled and filtered air then leaves filter 19 by way of line 22, having valve 23, and is passed into line 24 to be conducted therethrough to the subsequent steps of the process. At infrequent intervals of time, when filter 19 requires regeneration, valves 21 and 23 are closed and valves 25 and 26, in lines 27 and 28 respectively, are opened to divert the passage of the air from line 18 into lter 29.

For this regeneration, apart of the'lteredair-frontline 24is diverted therefrom through line 30 by openingV According to the present modification for controlling the difference between the temperatures of the compressed air and the nitrogen-rich product in exchanger 3, about 7% of the cooled and filtered air is diverted fromv line 24 through line 40, as controlled by valve 41. This diverted portion is passed through the separate, or unbalance, passage 42 wherein it is disposed in heat exchange relation with the colder parts of the reversing exchanger.

3. Thedirection of flow of this diverted portion of the cooled air is countercurrent to the ow of the compressed air being cooled in either passage 6 or 7 of the exchanger. As a result of its heat exchange with the compressed air, this diverted portion absorbs heat from the air so as to be warmed to about F. when it leaves passage 42 by way of line 43. The additional heat exchange relation provided by the diverted portion of the air passing through passageway 42, decreases the difference between the temperatures of the streams of compressed air and nitrogen-rich product toward the cold end of exchanger 3 to values which are less than the maximum allowable temperature difference. The resultant effect of passing the diverted portion of the cooled compressed air through,

passageway 42 is to increase the mass rate of ow of cold gaseous streams relative to the mass rate of ow of compressed air, thus unbalance the mass rates of flow and thereby to decrease the difference between the temperatures of the warm and cold streams toward the cold end of the exchanger. By this expedient of unbalance ow, it is possible to overcome the tendency of the higher speciiic heat of the compressed air to increase the temperature difference in the colder regions of the exchanger. Indeed it is possible to bring about as great a decrease in temperature difference as may be desired. By thus reducing the difference between the temperatures of the counterflowing streams in the colder regions of the exchanger to values less than the maximum allowable temperature difference for completely evaporating water and carbon dioxide deposits, there is no tendency for the exchanger to plug up and valves 2 and 9 may regulate the reversing cycles indefinitely.

Pursuant to another modification of the procedure to change the relative mass ows of warm and cold streams in exchanger 3,- Which is exemplified in the process arrangement shown by Figure 5, the difference between the temperatures of the compressed air and nitrogen-rich product in the colder parts of the exchanger may also be reduced. Referring now to Figure 5, according to this latter modification, a portion of the nitrogen-rich product is diverted from line 101d and passed through line 128 to passageway 42d of reversing exchanger 3d. The diverted portion is passed therethrough in countercurrent heat exchange to the compressed air stream in the colder portions of the exchangers whereby it is warmed. The thus warmed diverted portion is returned subsequently to line Mild as shall hereinafter be described.

Returning now to the explanation of Figure l, in all cases wherein the mass rates of ow of the countercurrent streams in heat exchange relation are unbalanced by the separate stream passed through passageway 42, the heat absorbed by the stream in passageway 42 exceeds the amount of heat necessary to warm the quantity of air about to be expanded to a temperature sufficiently high so that the possibility of liquid formation during expansion is obviated. The heat absorbed by the unbalance stream is in excess regardless as to whether a 9. gaseous or liquid oxygen-rich component is withdrawn as product of the separation, although the excess is smaller when a liquid product, is withdrawn.

The improved method of this invention utilizes the available heat absorbed by the unbalance stream to adjust the temperature of the portion of the cold compressed air about to be expanded. According to the process arrangement exemplified in Figure 1, approximately 17.5% of the cooled compressed air must be expanded in expansion engine 54. To obviate any possibility of liquid forming in the engine during this expansion, the air about to be expanded must enter the engine at 244 F. However, the cooled compressed air in line 24 has been reduced to a temperature of 273 F. in order to precipitate the maximum possible amount of its carbon dioxide constituent in exchanger 3. As stated heretofore, the unbalance stream leaves passageway 42 at 120 F. Therefore, this stream is at a suitable temperature so that a mixture of a part or all of it and a part of the cooled compressed air from line 24 may be made to provide for the desired amount of air and at the desired temperature for expansion. So, the warmed compressed air in line 43 is divided for the temperature readjustment. One of the divided portions, representing about 39% of the unbalance stream in line 43, is returned to line 24 by Way of by-pass line 44 having valve 45 positioned therein. The remaining 61% portion of the unbalance stream continues to flow in line 43 through valve 46 whereafter it is commingled with a regulated amount of the compressed air at 273 F. from line 24 which has been removed from the latter line at a point upstream from the point of introduction of the warm stream from line 44. The portion of cold compressed air used for temperature adjustment represents about 13% of the total incoming feed air and is passed from line 24 to line 43 by way of line 47 and valve 43.

The commingled stream in line 43 downstream from the inlet of line 47 attains a temperature of 244 F., the necessary temperature condition for gaseous phase expansion in this process. To prevent water from getting into the expansion engine during starting-up periods of operations, valve 49 can be closed and the compressed air diverted from line 43 through line 50, having valve S1, for passage through dryer 52. The dried air is then returned to line 43 by way of line 53. The drying medium may consist of any of the commercially wellknown agents, for example, silica gel. The compressed air enters expander 54 through valve 55, strainer 56 and valve 57. Valve 55 preferably is a solenoid valve which responds to the expander speed so as to prevent overspeeding of the engine. Expander 54 is mechanically connected to run in cooperation with an expander brake 58, which in the present instance is an electrical generator. The compressed air is expanded in the expansion engine to a pressure of 7.5 pounds per square inch gauge with the performance of external work. This results in a lowering of its temperature at the reduced pressure to about 305 F. The expanded air leaves expander 54 through line 59, and is conducted therethrough to section 6) of the fractionator 61 and introduced at an intermediate point for processing therein as hereinafter shall be described. l

Returning now to the cooled compressed air owing through line 24, Iafter the aforementioned 13% of air has been withdrawn from this line to line 47 and a part of the unbalance stream has been returned from line 43 by way of line 44, the temperature of the air in line 24 is then at about 266 F. The stream flowing through line 24 now amounts to approximately 82.5% of the cooled puried air which intially left reversing heat exchanger 3 through line 18. It undergoes heat exchange with the nitrogen-rich product in heat exchanger 62 t0 bring its temperature immediately above the liquefaction temperature of the air at the pressure of 76 pounds per square inch gauge, existing at this point in the system.

`a temperature of about 286 F.

10 The stream of air then leaves cooler 62 at a ytemperature of about 270 F. and is conveyed into the bottom of section 63 of fractionator 6ft 'by way of line 64, having pressure control valve 65 positioned therein.

It is the function of fractionator 61 to operate as a fractionating tower and separate the now cooled and purified feed lair into oxygen-rich and nitrogen-rich products by fractionation and rectification. For this purpose, tower 61 is separated into two compartments, or sections, 60 and 63. These sections operate at different pressures, the upper section 60 being under the lower pressure, and vbecause of this the section is termed the low pressure section while section 63 is designated as the high pressure section. The sections are provided with suitable means for promoting a plurality of intimate vapor-liquid contacts, which means may comprise fractionating trays provided with bubble caps. A calandria type heat exchanger 66 is positioned intermediately between the two sections. The calandria has the dual function of serving as the reiiux condenser for the high pressure bottom section 63 while simultaneously serving as the reboiler for the low pressure upper section 60. To allow calandria 66 to serve in its capacity for reboiling and condensing, the operating pressures in the low and high pressure sections are 'such that the temperature of the condensing vapors in the top of section 63 will be sufficient to transfer heat necessary to boil the liquid bottom product of the low pressure section 60. For this reason, the operating pressure in section 63 is held at approximately 71.5 pounds per square inch gauge while the operating pressure in section 60 is maintained at 7.5 pounds per square inch gauge.

The cooled but vaporous air from line 64 is introduced into section 63, preferably in the vapor space immediately under the bottom tray, and the rising vapors therefrom are brought into contact with descending liquid reflux on the trays of this section. In this manner, the air is separated into an oxygen-rich liquid bottom fraction having a temperature of about 279 F. and substantially pure liquid nitrogen top fraction. This top fraction supplies the liquid used for reflux in section 63 as obtained by condensation of the nitrogen vapors at in calandria 66. Valved drawoff lines 67 and 68 are connected to the bottom of section 63 and to the top of calandria 66 respectively, for use as drawoff lines in the event it becomes necessary to remove material from section 63 at these points.

The liquefied oxygen-rich product which collects in the base of section 63 is withdrawn therefrom in a regulated continuous stream through line 69, line 70 and valve 71 and thereafter introduced into lilter 72 which contains a body of suitable filtering or adsorbing material such as, for example, silica gel or activated carbon. lt is the purpose of lter 72 to remove any residual amounts of carbon dioxide or any other impurity, for example, acetylene, which may have penetrated thus far into the system. The oxygen-rich liquid, after the filtration step, is conveyed through line 73, valve 74 and line 75 to subcooler 76. In the event filter 72 should require revivitcation, valves 71 and 74 'are closed and valves 77 and 78 are opened to pass the liquid into and out of the alternate filter 79 by way of lines 80 and 81 respectively. The reviviiication of the filtering material of lilter 72 is accomplished by diverting a part of the cornpressed air from line 24 by way of line 30, thereafter warming this diverted portion and passing it into and out of the iilter through lines 82 and 83 lhaving valves S4 and 8S respectively. Filter 79 may likewise be revivitied when this iilter is olf the line by closing valves 77 and 78. In this case, the warmed air `from line 30 is passed into and out of the iilter by Way of lines 86 and 87, valves 88 and 89 respectively being opened. In subcooler 76 the stream of oxygen-rich liquid is cooled further bycold exchange with the nitrogen-rich vapors of the separation to such an extent that when this-stream is thereafter takenthrough line 90 -at atemperature of about 288 F. and expanded into the lower pressure section 60 ofthe fractionator through valve 91 the vaporization of this liquid is minimized.

Simultaneously with the passage of the oxygen-rich liquid through subcooler 76, the liquefied substantially pure nitrogen top product from the high pressure-section 63 is removed from the top tray 92 of that section at about 286 F. and passed through line 93 to subcooler 94. The stream of this liquid product of the primary separation likewise is subcooled by the cold vapors of the nitrogen-rich efiiuent from the topof the low pressure section 60 of the fractionator so that when this material subsequently is passed through line 95 at a temperature approximately 306 F. and expanded into the top of the low pressure lsection through valve 96, there is no excessive fiashing of this material. Rectificationof the expanded vaporous air from expander 54 and the components expanded through valves 91 and 96 takes place on the vapor-liquid contacting trays in section 60. The liquid bottom product of this rectification, being substantially pure oxygen, accumulates at a temperature of about 292 F. in a pool surrounding the tubes of Calandria 66. As stated, vaporization of the oxygen liquid is brought about as the result of condensation of nitrogen vapors within the tubes of this calandria to provide the reboiling vapor for section 60 and supply the product oxygen vapors which are removed from the fractionator at a point immediately above the level of the pool of liquefied oxygen through line 97. These vapors which are removed from fractionator 61 at a temperature of 292 F. are carried by way of line 97 to reversing exchanger 3 wherein they are conducted through the inner passageway 16 for the counter-current heat exchange with the incoming compressed air. Having thus given up their recoverable cold content to the air, the,

vapors of the oxygen-rich component are withdrawn from exchanger 3 by way of line 9S at about 83 F. and under an outlet pressure of about 3 pounds per square inch gauge.

The nitrogen-rich vapors are taken overhead from fractionator 61 through line 99 at a temperature approximately 312 F. and at 7.5 pounds per square inch gauge. These are brought into heat exchange in subccoler 94 with the liquefied nitrogen from line 93 whereby the latter has its temperature decreased, as stated, from 286 F. to 306 F. while the efiiuent nitrogenrich vapors are warmed to about 292 F. At this latter temperature 'the nitrogen-rich vapors are immediately passed through line 100 to subcooler 76 wherein they are further warmed to 283 F. in cooling the oxygen-rich liquid from line 75 from 279 F. to 288 F. The partially armed nitrogen-rich vapors are caused to fiow through line 101 to cooler 62 for a final heat exchange with compressed air before they are taken through line 102 to the check valve manifold controlling fluid flow into the cold end of reversing exchanger 3. In the event all of the nitrogen-rich vapors are not required for heat exchange` in heat exchanger 62, the unused portion is by-passed around the cooler through by-pass liney 103, having control valve 104, which connects between lines 101 and 102. in the present illustrative process, the nitrogen-rich vapors reach thecheck valve manifold at a temperature of about 278 F. after this latter heat exch During the period of time when reversing valves 2 and 9 are actuated to canse the compressed air stream to flow through passageway 6 of exchanger 3 and to leave the exchanger by way of line 17 and check valve 13, the check valves cause the backward-returning nitrogen-rich vapors to flow thro-ugh check valve 12 and lines 105 and 106 for passage through passageway 7 ofreversing exchanger 3. Having been warmed by its countercurrent heat exchange with the compressed'air in passageway 6 to a temperature of about 83 F., the nitrogen-rich vapors are then withdrawn from the reversing exchangerand from the system through lines 5 and 8, reversing valve 9 and line 10. During the opposite phase in the operation of reversing exchanger 3, that is, when reversing valves 2 and 9 are actuated to cause the compressed air to iiow through passageway 7 and to leave the reversing exchanger by way of line 106 and check valve 14, the check valves automatically actuate themselves to permit the cold nitrogen-rich vapors to flow from line 102 through check valve 11 and line 17 for passage through passageway 6. After this, the warmed nitrogen-rich vapors are vented from the system through lines 4 and 15, reversing valve 9 and line 10.

Referring again to the warm compressed air lstream flowing through line 43 from the unbalance passage 42, according to an alternate embodiment of the invention, a portion of this stream is returned at a temperature of 120 F. to line 24 through line 107 in an amount controlled by valve 108 and commingled with the main cold compressed air stream in line 24 before this cold stream again is redivided to supply the necessary cold air required to augment the warmed stream flowing through valve 46 in line 43 to the amount and temperature needed at the expansion engine inlet. When the operation of the gas separating system is in accordance with this embodiment, valve 45 in line 44 is closed.

Another alternate embodiment for obtaining the necessary amount of compressed air at the proper temperature for expansion is illustrated in Figure 2. In this figure, only the apparatus of Figure l is shown as is necessary for the understanding of this modification and those parts which are identical to parts shown in Figure l are shown with the same numerals bearing the subscript a. In the modification of Figure 2 the compressed air is cooled to the same temperature in the reversing heat exchanger as for the case shown in Figure l and now flows through line 24a at a temperature of 273 F. The unbalance stream temperature in line 43a is 120 F. as in the former example and in the present modification a portion of this latter stream also is returned to line 24a. In this case, however, by passing the return portion through exchanger 62a by way of lines 109 and 110, a part of the heat content of the portion is dissipated to the nitrogenrich vapors passing into and out of exchanger 62a by lines 101a and 10211 at temperatures of 283 F. and 278 F. respectively. The stream passing into line 24a from line 110 is still sufficiently warm to raise the temperature of the air in the former line from 273 F. to 270 F. At this latter temperature, the compressed a1r is passed through line 24a directly to the fractionator. Simultaneously, with the foregoing method of adjusting the temperature of the air fiowing through line 24a by the return of a part of the warm unbalance stream in line 43a, the remaining part of the latter stream is adjusted in volume and temperature to conditions satisfactory to supply the refrigeration requirements of the system by a vapor phase expansion. For this adjustment, a part of the air in line 24a is diverted therefrom at a point upstream to the inlet connection of line 110 and is transferred at 273 F. through line 47a in an amount as controlled by valve 48a to line 43a to a point downstream from valve 46a. The commingling of the two fiuids is controlled so that the desired amount of compressed air at thedesired temperature flows through line 43a to the inlet end of the expander.

A further modification of the processing flow arrangement of Figure l is illustrated in Figure 3 to show another embodiment wherein the heat content of thermbalance stream may be utilized to supply the'desired amount of compressed air tothe expander at a temperature which prohibits liquefaction in the engine. Figure 3 likewise contains only the essential parts of Figure 1 which are necessary for the understandingof this'modi- 13 cation and the parts of this ligure which are identical to parts shown in Figure 1 bear the same numerals with the subscript b. In the present modification the compressed air in line 24b has been cooled in the reversing exchanger to 270 F. which is the temperature at which it passes to the fractionator. The unbalance stream in line 43b passes therethrough at 120 F. as in each of the previous modications. In the present case, however, essentially the total stream in line 43h is passed through exchanger 62b for heat exchange with the nitrogen-rich vapors and, as the result, these vapors which enter the exchanger through line 101b at 283 F. are warmed to 275 F. when they leave by line 10211 on their way to the reversing exchanger. Since air in line 43b is insuicient in quantity to provide for the refrigeration requirements of the system, it must be augmented with further amounts of air from line 24h. For this reason, the temperature of the air leaving exchanger 62b through valved line 112 is at about 210 F. Then when it is mixed with cooled air entering line 112 from line 24b through valved line 113, the resultant temperature of the stream of air passing on to the expander is at about 244 F. which is suciently high to prevent liquefaction in the engine. In the event it is necessary for control purposes, part of the unbalance stream in line 43]) may by-pass exchanger 62b through valved line 114. For similar reasons, a part of the partly cooled unbalance stream in line 112 may be injected into line 24b by means of the valved line 115.

In Figure 4 a modification is shown wherein the desired quantity of compressed gas is passed tothe expansion step at a temperature which precludes the formation of liquefaction during expansion when the compressed air leaves the reversing heat exchanger at a temperature below that desired at its point of introduction to the fractionation step. For example, in the event air is cooled to 273 F. in the exchanger to ensure substantially complete vprecipitation of carbon dioxide and the air is passed into the fractionator at 270 F., the excess cold may be effectively recovered by heat exchange with the stream passingto the expansion step. Figure 4 shows only that part of the liow arrangement of Figure 1 which is necessary to the understanding of this modification and the parts identical to those of Figure 1 are shown with the same numerals bearing the subscript c. In the present modiication a portion of the cooled air passing from the reversing heat exchange step through line 24C, in a quantity suitable for expansion with the unbalance stream, is combined with the unbalance stream is line 43e by way of valved line 47e. The combined stream, since it is still too warm for eflicient expansion, is then subjected to further cooling rst in exchanger 116 by heat exchange with the remaining part ofthe air in line 24e with the resultant warming of this latter part to 270 F.1when it is subsequently passed to the fractionator by Way of line 117. The thus partly cooled stream for expansion leaves exchanger 116 through line 11S and is adjusted to the desired temperature for expansion in exchanger 119 by heat exchange with the cold nitrogen-rich vapors. These vapors are at about 285 F. as they pass into exchanger 119 through line 101e` and are warmed therein so that they pass to the reversing exchanger at a temperature of about 278 F. which temperature is suitable for their further heat exchange with the incoming compressed air. The now cooled stream for expansion passes to the expander from exchanger 119 through line 120 at the desired temperature of about 244 F. Valved lines 121, 122 and 123 are by-pass lines in the event it becomes necessary to control conditions in exchangers 116 and 119. It is understood that exchangers 116 and 119 need not necessarily b e positioned in the described order since the air owing to the expander may be made to owtirst through exchanger 119 and then through exchanger 116. Further, V`it is not essential to employ 'two separate exchangers for the heatY exchange performed by exchangers 116 and 119, particularly when extended surface similar to that used in exchanger 3 of Figure 1 is employed,V since the total heat exchange relation may be accomplished by passing the individual streams through separate passages of a single exchanger Vessel. It may be desirable in some cases to carry out the heat exchange of exchangers 116 and 119 before combining the portion of compressed air from line 24e with the unbalance stream in line 43e. In this event line 47 will connect with line 120.

In the event a part of the cold nitrogen-rich vapors is employed in place of the compressed air for unbalancing the heat exchange relation eiected in the reversing exchanger, the modified flow arrangement as shown in Figure 5 will replace that shown in Figure 1. Inasmuch as the essential pieces of equipment in Figure 5 are for the most part identical to those of Figure 1, identical parts in Figure 5 are shown with the same numerals bearing the subscript d. Referring to Figure 5, for an example to describe this modification, consider that atmospheric air which previously has been treated to remove dust, water or other impurities, such as acetylene, and which has been compressed to about pounds per square inch gauge, is introduced through line 1d at a temperature of about 90 F. Line 1d connects with reversing valve 2d which has settings to permit passage of the compressed air into reversing exchanger 3d, either by way of line 4d or line 5d. Since the system involving exchanger 3d is operated in accordance with the method described in connection with Figure 1, a repetition of the description and operation of this or other parts of the apparatus described in connection with Figure l is unnecessary. In the present example, however, the compressed air leaving through line 18d now has been cooled in the exchanger to a temperature of 263 F. The major portion of the cooled air, after passage through filter 19d, or alternatively filter 29d, is introduced at this temperature into high pressure section 63d of fractionator 61d by way of line 24d. Simultaneously, a minor portion of the air, which in this event amounts to about 22.5 percent of the total compressed air, is diverted from line 24d and passed through line 125 to heat exchanger 126. In this vessel the temperature of the diverted portion is increased to about 235 F. by a heat exchange relation as shall hereafter be explained. The thus warmed air then is passed by Way of line 127 to expander 54d under a pressure of 87 pounds per square inch gauge, having lost approximately 3 pounds of pressure because of friction drop in the apparatus. Expander 54d is employed to reduce the pressure of this portion of the air to 9 pounds per square inch gauge with the performance of external work and, as the result of the pressure reduction, a reduction in temperature also occurs so that the air reaches the low pressure section 60d of fractionator 61a' by way of line 59d at a temperature of about 304 F. Rectication of the air occurs in fractionator 61d in accordance with the method of operation described in connection with Figure 1. In the present example, however, the cold vaporous nitrogen-rich product passes overhead from fractionator 61d through line 99dat a temperature of 310 F. and under a gauge pressure of 9 pounds per square inch. This temperature is subsequently increased to about 280 F. as the nitrogen vapors pass through subcoolers 94d and 76d. Friction losses also change the pressure of these vapors to about 7 pounds per square inch gauge. A part of the nitro- Vgen-containing vapors is diverted from line 101a' at these p ducedat thistemperature into .heat exchanger.z 126.- Exe Vthis temperature, it is passed through line 130 and returned to the main nitrogen stream in line ltlld at a point downstream from the point of its initial diversion. The commingling of the warmed portion from line 130 with the main nitrogen stream in line 10M increases the temperature of the combined stream passing to exchanger 3d to a temperature of about 271 F. Thus, the portion of the compressed air which is to be subjected to expansion is diverted directly from the purified stream of cooled compressed air as it leaves the reversing heat exchanger, warmed to the desired pre-expansion temperature by heat exchange with the eiuent stream from the imbalance passage of the heat exchange and passed directly to the expansion step.

It may be desirable, under certain conditions, to divide the unbalance passage of the heat exchanger into two sections to permit the simultaneous employment therein of -a part of both the stream of incoming compressed gaseous feed and a product stream rich in one of the separated components. In such case, the compressed gas for expansion can be obtained at the desired temperature preliminarily to expansion in accordance with any, or combination, of the heretofore described methods. Furthermore, a portion of the compressed air to be expanded, can bediverted through an unbalance path of the heatexchange zone, warmed therein to the necessary temperature andvpassed directly to the expansion step. Alternatively, that portion of compressed air diverted through the unbalance passage can be warmed to a substantially high temperature and subsequently cooled, for example by heat exchange with the main stream of cooled compressed air, and passed to the expansion step either alone or commingled with a further portion of cooledcompressed air.

It is understood that the present invention is not limited to any of the embodiments described herein for Villustrative purposes, nor to the specic separation of air as a gaseous mixture but only in and by the following claims.

We claim:

l. In the method of treating air wherein a stream of compressed air, containing carbon dioxide as an impurity, is passed in one direction of ilow through a reversing heat exchange zone in heat exchange relation with relatively cool counterowing fluid, obtained from the air in Va later stage of treatment and not greater in mass quantity than said stream of air, to eect cooling of the air and resultant precipitation of the impurity in a colder portion of the heat exchange zone, and wherein the stream of air and a second gaseous stream comprising at least a portion of said counterowing tiuid and substantially free of the impurity and at lower temperature than said colder portion are alternated one with the other in two paths of the heat exchange zone whereby the second gaseous stream passes over precipitated impurity and evaporates it, and further wherein a part of the stream of cooled air is subjected to expansion with production of external work and another part is subjected to subsequent treatment in compressed condition; the improvement which comprises cooling the stream of compressed air in the heat exchange zone to a temperature below the precipitation temperature of the impurity, removing the stream of cooled air from the heat exchange Zone, passing Va uid stream of cooled air, in countercurrent Vheat exchange with said stream of compressed air through a separate path in the heat :exchangezzone disposed in heat exchange relation withY at least a pantofsaidA colder. portion whereby said fluid streamis warmed by additionally cooling said stream ofcompressedv air to maintain a difference between the temperatureat which the precipitation of impurity occurs at any point in said colder portion and the temperature atwhich said second gaseous stream passes over precipitated impurity which is less than would exist but for the passage of saidA iiuid stream through the separate path, removing the iiud stream from the separate path, passing a fraction thereof in heat exchange with at least a part of said second gaseous stream passing to the reversing heat exchange zone and` cooling the fraction sufficiently to provide for an average temperature when said fraction is subsequently combined with a second fraction of said uid stream which average temperature is suitable for effecting vapor phase expansion of the combined fractions, combining said fractions thereby obtainingl saidy average temperaturer and producing the combined fractions as Vair to be` subjected to expansion and then expanding withproduction of external work.

2. In the method of treating` air wherein a stream of compressed air, containingcarbon dioxide as an impurity, is passed in one direction of flow through a reversing heat exchange zone in heat exchange relation with relatively cool counterowing liuid, obtained from the air in a later stage of treatment and not greater in mass quantity than said stream of air, to elect cooling of the air and resultant precipitation of the impurity in a colder portion of thel heat exchange zone, and wherein the streamofairand a second gaseous stream comprising at least a portion of s aid counterowing fluid and substantially free of the impurity andy at lower temperature than said colder portion are alternated one with the other in two paths of the heat. exchange zone whereby the second gaseous stream passes over precipitated impurity and evaporates it, and further wherein a part of the stream of cooled airfis subjected to expansion with production of external work and another part issubjected to subsequent treatment in compressed condition; the improvement which comprises cooling the stream of compressed air in the heat exchange zone to a temperature below the precipitation temperature of the impurity, removing the stream of cooled air from the heat exchange zone, passing a fluid stream of cooled air, in countercurrent heat exchange with said stream of compressed air through a separate path in the heat exchange zone disposed in heat exchange relation with at least a part of said colder portion whereby said fluid stream is warmed by additionally cooling said stream of compressed air to maintain a difference between the temperature at which the precipitation of impurity occurs at any point in said colder portion and thegtemperature at which said second gaseous stream passes over precipitated impurity which is less than wouldexist but forthe passage of said tluid stream through the separate path, removing the tluid stream from the separate path, passing a fraction thereof in heat exchange with at least a part of said second gaseous stream passing to the reversing heat exchange zone and cooling the fraction sufliciently to provide for an average temperature when said fraction is subsequently combined with a second fraction of said uid stream and another portion of said stream of cooled air, which average temperature is suitable for electingrvapor phasel expansion of the combined fractions, combining the fractions with the last-mentioned portion thereby obtaining said average .temperature and producing said part of the stream of cooled air to be subjected to expansion and then expanding the thus produced part with production of external work.

3. In the method oftreating air wherein a stream of compressed air, containing carbondioxide as animpurity, is passed in one direction of ow through a reversing heat exchange zone v in heat exchange relation with relatively .cool counterflowing uid, obtained from the air in a later sas-10,994

stage of treatment and not greater in mass quantityrthanA said stream of air, to eiect cooling of the air and resultant precipitation of the impurity in a colder portion of the heat exchange zone, and wherein the stream of air and a second gaseous stream comprising at least a portion of said counterflowing fluid and substantially free of the impurity and at lower temperature than said colder portion are alternated one with the other in two paths of the heat exchange zone whereby the second gaseous stream passes over precipitated impurity and evaporates it, and further wherein a part of the stream of cooled air is subjected to expansion with production of external work and another part is subjected to subsequent treatment in compressed condition; the improvement which comprises cooling the stream ot compressed air in the heat exchange zone to a temperature below the precipitation temperature of the impurity, removing the stream of cooled air from the heat exchange zone, passing a fluid stream of cooled air, in countercurrent heat exchange with said stream of compressed air through a separate path in the heat exchange zone disposed in heat exchange relation with at least a part of said colder portion whereby said fluid stream is warmed by additionally cooling said stream of compressed air to maintain a difference between the temperature at which the precipitation of impurity occurs at any point in said colder portion and the temperature at which said second gaseous stream passes over precipitated impurity which is less than would exist but for the passage of said uid stream through the separate path, removing the uid stream from the separate path, passing a fraction thereof in heatv exchange with at least a part of said second gaseous stream passing to the reversing heat exchange zone and cooling the fraction sutciently to provide for an average temperature when said fraction is subsequently combined with a second fraction of said iluid stream which average temperature is suitable for effecting vapor phase expansion of the combined fractions, mixing a portion of the combined fractions with the part of the stream of cooled air to be subjected to subsequent treatment whereby the other portion of the combined fractions rernains as air to be subjected to expansion and then expanding with production of external work.

4. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inflowing charge stream of said compressed gaseous mixture enters said system at a pre-expansion pressure from a reversing heat exchange zone in which said inflowing stream is cooled, and in a cold part of which, high-boiling impurities are precipitated, and wherein at least one outflowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange Zone, absorbing heat and scavenging said precipitated high-boiling impurities by revaporization, said iniowing and outowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, and wherein a portion of gas under pre-expansion pressure is expanded to post-expansion pressure in an expansion engine to provide refrigeration, a method for carrying out said expansion in said expansion engine at low temperatures without condensation of liquid or precipitation of residual high-boiling impurities during said expansion, which includes the steps of: introducing outowing product from said fractionating'system into the cold end of said reversing heat exchange zone at temperatures sufficiently low to precipitate substantially all high-boiling impurities from said inflowing compressed mixture within said reversing heat-exchange zone, but not so low as to cause condensation of said inowing mixture within said reversing heat exchange zone; withdrawing a stream of gas from said fractionating system and continuously flowing said stream through a non-reversing path in said reversing heat exchange zone, in countercurrent heat exchange with said inowing mixture along said cold part in which high-boiling impurities are precipitated, and in suilcient quantity to unbalance the counterowing streams by maintaining a total mass flow of outowing streams through said cold part in excess of the mass flow rate of said infiowing mixture, where-I by the excessive accumulation of impurities precipitated from said inflowing mixture within said cold part is prevented, and said unbalancing stream is warmed to a temperature substantially higher than the minimum temperature` at which gas can be introduced into said expansion engine without condensation during expansion; returning said warmed unbalancing stream to said fractionating system; and cooling at least a part of said returned unbalancing stream by heat exchange with a stream in said tructionating system flowing to said expansion engine; and simultaneously warming the stream entering said expansion engine to a temperature at which expansion can be effected without condensation Within said expansion engine.

5. In a pro-cess for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inilowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure from a reversing heat exchange zone in which said inowing stream is cooled, andina cold part of which, high-boiling impurities are precipitated, and wherein at least one outowing product stream leaves said system at a post-expansion pressure thro-ugh said reversing heat exchange zone, absorbing heat and scavenging said precipitated high-boiling impurities by revaporization, said inilowing and outowing streams being owed countercurrently and alternately with each other through periodically reversinggpaths in a heat exchange relation in said reversing heat exchange zone, and wherein a portion of gas under pre-expansion pressure is expanded to post-expansion pressure in an expansion engine to provide refrigeration, a method for carrying out said expansion in said expansion engine at low temperatures without condensation of liquid or precipitation of residual high-boiling impurities during said expansion, which includes the steps of: introducing outflowing product from said fraction-ating system into the cold end of said reversing heat exchange zone at temperatures sufliciently low to precipitate substantially all high-boiling impurities from said inflowing compressed mixture within said reversng heat exchange zone, but not so low as to cause condensation of said inilowing mixture within said reversing heat exchange zone; diverting a portion of said product stream outowing from said fractionating system before it enters said reversing heat exchange zone and continuously owing said stream through a non-reversing path in said reversing heat exchange Zone, in countercurrent heat exchange with said inflowing mixture along said cold part in which high-boiling impurities are precipitated, and in sufficient quantity to unbalance the counterowing streams by maintaining a total mass ow of outilowing streams through saidcold part in excess of the mass ow rate of said inowing mixture, whereby the excessive accumulation of impurities precipitated from said inflowing mixture within said cold part is prevented, and said unbalancing stream is warmed to a temperature substantially higher than the minimum temperature at which gas can be introduced into said expansion engine without condensation during expansion; returning said warmed unbalancing stream to said tractionating system; and flowing at least a part of said returned unbalancing stream in indirect heat exchange with a stream flowing in said fractionating system to said expansion engine; and returning said unbalancing stream to said outflowing product at a point downstream from the initial diversion.

6. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure from a reversing heat exchange zone in which said inowing stream is cooled, and in a cold part of which, high-boiling impurities are precipitated,- and wherein at least one outowing product stream leaves said system at apost-expansion pressure through said reversing heat exchange zone, absorbing heat and scavenging said precipitated high-boiling impurities by revaporization, said inilowing and outowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, and wherein a portion of said gas entering said system under pre-expansion pressure is expanded to post-expansion pressure in an expansion engine to provide refrigeration, a method for carrying out said expansion in said expansion engine at low temperatures without condensation of liquid or precipitation of residual high-boiling impurities during said expansion, which includes the steps of: introducing outowing product from said fractionating system into the cold end of said reversing heat exchange zone at temperatures suiliciently low to precipitate substantially all high-boiling impurities from said inowing compressed mixture within said reversing heat exchange zone, not so low as to cause condensation of said inowing mixture within said reversing heat exchange zone; returning a stream of gas under pre-expansion pressure from said fractionating system and continuously ilowing said stream through a non-reversing path in said reversing heat exchange zone, in countercurrent heat exchange with said inilowing mixturealong said cold part in which highboiling impurities are precipitated and in sucient quantity to unbalance the countercurrent stream by maintaining a total mass flow of outowing streams through said cold part in excess of the mass ilow rate of said iniowing mixture, whereby excessive accumulation of impurities precipitated from said inowing mixture within said cold part is prevented, and said unbalancing stream is warmed to a temperature substantially higher than the minimum temperature at which gas can be introduced into said expansion engine without condensation during expansion; returning said warmed unbalancing stream to said fractionating system; combining a part of said unbalancing stream with a relatively colder stream owing in said fractionating system under pre-expansion pressure to make up a quantity of gas required to be expanded in said expansion engine to supply refrigeration to said system, and in proportions to produce a stream at a temperature suiciently high to effect said expansion without condensation; and returning the remainder of said warmed unbalancing stream to said fractionating system at a point from which it can not ow through said expansion engine.

7. In a process for fractionating a compressed gaseous mixture in-a low-temperature expansion and fractionating system, wherein an iniiowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure from a reversing heat exchange zone in which said inowing stream is cooled, and in a cold part of which, high-boiling impurities are precipitated, and wherein at least one outlowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone, absorbing heat and scavenging said precipitated high-boiling impurities by revaporization, said intlowing and outtlowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, and wherein a portion of said gas entering said system under pre-expansion pressure is expanded to post-expansion pressure in an expansion engine to provide refrigeration, a method for carrying out said expansion in'said expansion engine at low temperatures without condensation of liquid or precipitation of residual high-boiling impurities during said expansion, which includes the steps of: introducing outowing product from said fractionating system into the cold end of said reversing heat exchangezone at temperatures sufficiently low to precipitate substantially all high-boiling impurities from said iniiowing compressed mixture within said reversing heat exchange zone, but not so low as to cause condensation of said inowing mixture within said reversing heat exchangezone; separating said compressed gaseous mixture entering said fractionating system from said reversing heat exchange zone into at least three streams, one stream being returned to4 said reversing heat exchange zone to imbalance the mass ow rates of countertlowing streams therein, one being owed to said expansion engine, and one being owed to fractionation; flowing said unbalancing stream through a non-reversing path in said reversing heat exchange zone, in countercurrent heat exchange with said inowing mixture along said cold part in which high-boiling impurities are precipitated, and in sutticient quantity 4to maintain a total mass ow of outflowing streams through said cold part in excess of thc mass ow rate of said inowing mixture, whereby the excessive accumulation of impurities precipitated from said inflowing mixture within said cold part is prevented, and said unbalancing stream is warmed to a temperature substantially higher than that which would produce condensation during expansion of said gas to post-expansion pressure; returning said warmed unbalancing stream to said fractionating system and combining a part thereof with said relatively colder expansion engine stream to make up a quantity of gas required to be expanded in said expansion engine to supply refrigeration to said system, and in proportions to produce a stream at a temperature sufliciently high to effect said expansion without condensation; and combining the remainder of said warmed unbalancing stream with said stream owing to fractionation.

8. A process for producing oxygen by the liquefaction' and `rectification of air in a two stage rectification system involving a high and a low pressure stage which comprises passing a stream of air through a path in a heat exchange zone, passing from the low pressure stage of the rectication system a stream of oxygen through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, withdrawing aV minor portion of the air stream leaving said heat exchange zone, withdrawing a minor portion of the nitrogen stream leaving the low pressure stage of said rectication system, warming one of said minor streams by passage through said heat exchange zone, passing the thus warmed stream in heat exchange relation with the other minor stream to warm it lthereby producing warmed minor streams of air and nitrogen, said warmed minor air stream being at a temperature such that upon subsequent expansion substantially no liquid air is formed, mixing the warmed minor nitrogen stream with the remaining major portion of the nitrogen withdrawn from the low pressure stage of the rectification system to produce a nitrogen stream at a temperature of about 5 F. below the temperature of the air `leaving said zone, passing the resulting nitrogen stream through still another path in Ysaid heat exchange zone in heat exchange relation with the other streams passing through said heat exchange zone, thereby cooling the air leaving said zone to a temperature close to its condensation point at the pressure prevailing in said zone and effecting substantially complete removal of carbon dioxide from said air stream, expanding at least the warmed minor air stream to produce refrigeration in amount sufficient to compensate for cold losses resulting from difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, introducingthe expanded air into the low pressure stage of the rectification system, passing the major portion of the air leaving said heat exchange zone to the high pressure Astage of said rectification system and periodically reversing the Aow 'of air and nitrogen through their re- 21 spective paths in said zone so that upon each of said reversals the nitrogen substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

References Cited in the le of this patent UNITED STATES PATENTS 22 Trumpler Feb. 8, 1949 Garbo July 4, 1950 Garbo Jan. 9, 1951 Jenny Dec. 25, 1951 Garbo June 9, 1953 OTHER REFERENCES Chemical Engineering, March 1947, Air separation principles and technology, pp. 126-134.

UNITED STATES PATENT oEEToE CERTIFICATE 0F CORRECTION Patent Nou 2,840,994

. July l, 1958 Walter En Lobo et al,

It is hereby certified is of' 'the above numbered patent Patent should read as corrected below.

Column 5, line 5l, for "consideration" read w concentration =IEI=,f column 8, line 63, for "exchangers" reed m ex l changer lm; column 13, line 61, for "@2850 E," read m l@5,2830 E., @n

Signed and sealed this 9th day of September 1958.,

SEAL) ttest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oicer Commissioner of Patents 

1. IN THE METHOD OF TREATING AIR WHEREIN A STREAM OF COMPRESSED AIR, CONTAINING CARBON DIOXIDE AS AN IMPURITY, IS PASSED IN ONE DIRECTION OF FLOW THROUGH A REVERSING HEAT EXCHANGE ZONE IN HEAT EXCHANGE RELATION WITH RELATIVELY COOL COUNTERFLOWING FLUID, OBTAINED FROM THE AIR IN A LATTER STAGE OF TREATMENT AND NOT GREATER IN MASS QUANTITY THAN SAID STREAM OF AIR, TO EFFECT COOLING OF THE AIR AND RESULTANT PRECIPITATION OF THE IMPURITY IN A COLDER PORTION OF THE HEAT EXCHANGE ZONE, AND WHEREIN THE STREAM OF AIR AND A SECOND GASEOUS STREAM COMPRISING AT LEAST A PORTION OF SAID COUNTERFLOWING FLUID AND SUBSTANTIALLY FREE OF THE IMPURITY AND AT LOWER TEMPERATURE THAN SAID COLDER PORTION ARE ALTERNATED ONE WITH THE OTHER IN TWO PATHS OF THE HEAT EXCHANGE ZONE WHEREBY THE SECOND GASEOUS STREAM PASSES OVER PRECIPITATED IMPURITY AND EVAPORATES IT, AND FURTHER WHEREIN A PART OF THE STREAM OF COOLED AIR IS SUBJECTED TO EXPANSION WITH PRODUCTION OF EXTERNAL WORK AND ANOTHER PART IS SUBJECTED TO SUBSEQUENT TREATMENT IN COMPRESSED CONDITION; THE IMPROVEMENT WHICH COMPRISES COOLING THE STREAM OF COMPRESSED AIR IN THE HEAT EXCHANGE ZONE TO A TEMPERATURE BELOW THE PRECIPITATION TEMPERATURE OF THE IMPURITY, REMOVING THE STREAM OF COOLED AIR FROM THE HEAT EXCHANGE ZONE, PASSING A FLUID STREAM OF COOLED AIR, IN COUNTERCURRENT HEAT EXCHANGE WITH SAID STREAM OF COMPRESSED AIR THROUGH A SEPARATE PATH IN THE HEAT EXCHANGE ZONE DISPOSED IN HEAT EXCHANGE RELATION WITH AT LEAST A PART OF SAID COLDER PORTION WHEREBY SAID FLUID STREAM IS WARMED BY ADDITIONALLY COOLING SAID STREAM OF COMPRESSED AIR TO MAINTAIN A DIFFERENCE BETWEEN THE 